Water Quality in the Pirita River, a Raw Water Source for Tallinn

Proc. Estonian Acad. Sci. Biol. Ecol., 2001, 50, 4, 269–278 WATER QUALITY IN THE PIRITA RIVER, A RAW WATER SOURCE FOR TALLINN Rein RANNAMÄEa and Ingeborg VELDREb a Health Protection Service of Harjumaa and Tallinn, Narva mnt. 48, 10150 Tallinn, Estonia b Institute of Experimental and Clinical Medicine, Hiiu 42, 11619 Tallinn, Estonia Received 2 March 2001, in revised form 11 June 2001 Abstract. The quality of the water of the Pirita River was evaluated on the ground of organoleptical, chemical, and microbiological analyses. Background values of some water quality indicators in the Pirita River were often higher than in the Pirita–Ülemiste Canal. The oxygen regime in the river was favourable ensuring the required conditions for self-purification. The BOD7/PC ratio of the river water was low, which shows that natural biochemically stable substances dominate in the water. Phenols in the river water were of vegetable origin. The concentrations of oil products and detergents in the river water were at the level of the sensitivity of the relevant determination methods. From the point of view of environmental health the raw water of the Pirita River is safe to be directed into the distribution system of the Tallinn Water Treatment Plant. Key words: river water, raw water, water quality, drinking water. The water supply of the city of Tallinn is based on surface and ground water with 91% of the water consumption satisfied by surface water. The average water consumption of an inhabitant of Tallinn was 120.3 L/day in 1999 and 133 L/day in 2000. The system of surface water supply of Tallinn includes the drainage basins of the upper reaches of the Pirita, Jägala, Soodla, and Pärnu rivers, the reservoirs on these rivers (Soodla, Paunküla, and Vaskjala), and also lakes Ülemiste and Raku (Fig. 1). The drainage basin of Pirita–Ülemiste covers about a third of the territory of Harju County, which raises additional requirements to the environmental protection of that region. The surface water is collected in Vaskjala Reservoir behind the dam of the Vaskjala hydroelectric power station. Further, the water moves through the Pirita–Ülemiste Canal to the treatment plant, situated on the north shore of Lake Ülemiste. The quality of the treated water must satisfy the general requirements of the Estonian drinking water standard (Eesti Standard, 1995). The higher the quality of the raw water entering 269 Surface water sources Tallinn. Res – of water reservoir. Surface Fig. 1. 270 the system, the higher is the quality of treated water and the lower are the costs of the treatment process to achieve a high quality of water. In addition to making the treatment process more expensive, low quality of raw water may cause problems in supplying the city with high quality water. The quality of drinking water, as well as the surface water sources of Tallinn, is investigated periodically. However, the quality of water in the Pirita River basin has not been thoroughly studied after 1994. Since then the amounts of agricultural waste products have diminished, but technical services and working regimes of sewage purification systems have deteriorated and therefore the effectiveness of the systems has decreased. The motivations for studying the quality of raw water in the Pirita River were the following: – The low quality of raw water demands additional quantities of coagulants for the treatment process; – The condition of water pipes is deteriorating and the velocity of water flow is decreasing. Water consumption has fallen and the water stays in the pipes for a long time; – The increasing doses of coagulants activate corrosion in the water pipes when two-valent iron is oxidizing into three-valent iron, which is then deposited in sediments. The colour and turbidity of water are rising; – Complaints of consumers are becoming more frequent. In our study we were interested in the quality of the water of the Pirita River, particularly as concerns its colour, concentration of nitrogen compounds, and content of organic substances. These indicators make it possible to evaluate the anthropogenic pollution with sewage as well as the influence of natural processes on the quality of river water. The aim of our study was to evaluate the quality of raw water and, if necessary, suggest improvements. MATERIAL AND METHODS The study was carried out from May to September 1999 and in April 2000. In 1999 water samples were taken five times (once each month) and in April 2000, three times. The water samples were taken from Paunküla Reservoir (site I) to Vaskjala Reservoir (site VII), and from the Pirita–Ülemiste Canal (site VIII) (Fig. 2). Although analysis of water from sites II–VI revealed some changes in the river water quality, it is most important to evaluate its quality when it enters Lake Ülemiste. Comparison of sites I, VII, and VIII makes it possible to estimate changes in the water quality from the background data in the Pirita River to Lake Ülemiste. The necessary amount of water for analyses was taken from the Pirita River and the Pirita–Ülemiste Canal in the current at a depth of 20–30 cm from the water surface by a skilled person. The water samples were analysed at a scheduled time, after which they were not preserved. 271 The Pirita River with sampling sites. sites. sampling with Pirita River The Fig. 2. 272 In all water samples organoleptical, chemical, and microbiological parameters were studied. All the tests were chosen according to the Estonian drinking water standard (Eesti Standard, 1995). Colour, odour, and transparency were tested organoleptically. Chemical analyses were used to assess the pH value, soluble –1 oxygen, permanganate consumption (in mg O2 L KMnO4), biochemical oxygen demand (BOD7), ammonia, nitrites, and nitrates. Microbiological tests were applied to detect coliform bacteria, thermotolerant coliform bacteria, and heterotrophic bacteria. All the organoleptical and chemical analyses were carried out in the Central Chemical Laboratory and the microbiological analyses in the Central Micro- biological Laboratory of the Health Protection Inspectorate in accordance with the analytical methods of ISO (International Organisation for Standardisation) standards accepted in Estonia. Concentrations of oil products and phenols were analysed in the Estonian Central Laboratory of Environmental Investigations. Oil products were extracted from water with n-pentane and analysed gas chromatographically. Their concentration was determined on a Japanese spectrofluorometer RF-540 Shimadzu at a synchronized scanning regime. The sum of oil products includes non- or little polarized hydrocarbons – aliphatic, acyclic, and aromatic hydrocarbons, which are soluble in n-pentane and represent the main part of oil products. High performance liquid chromatography was applied for the determination of one-basic (volatile) and two-basic (non- volatile) phenols in water. The presence and quantity of detergents were determined by the extraction– colorimetric method with methylene blue. The sensitivity of the method was 0.05–0.1 mg L–1. RESULTS AND DISCUSSION The chlorination process, usually applied to disinfect drinking water, is accompanied by formation of trihalomethanes (THMs), mainly chloroform. THMs are dangerous to humans: IARC qualifies chloroform as a possible carcinogen for humans of group 2B (WHO, 1987). THMs are formed by chemical interaction between chlorine and natural organic materials in water (Vogt & Regli, 1981). The precursors of chloroform remain largely unknown, but humic and fulvic acids are considered to be major precursors (Bedding et al., 1983). Analysis of chloroform was not included in our study because concentrations of THMs in Lake Ülemiste have been formerly investigated (Tammepuu, 1992). The chloroform content in the water of Lake Ülemiste was found to be far below the permissible level, the samples of chlorinated water containing from 11.8 to 97.8 J L–1 of chloroform. Studying different stages of water treatment processes Tammepuu found that the concentration of chloroform depends on the character of raw water, treatment technology, and methods of chlorination. The doses of 273 chlorine as well as the time of contact are important. The chloroform concentrations in drinking water varied in different seasons. The highest concentrations were found in summer – about 2–3 times higher than the permissible level of THMs at that time, which was 30 J L–1 till 1993. In 1993 WHO recommended 200 J/–1 as permissible level for chloroform. Since 1997, when the Tallinn Water Treatment Plant introduced ozonation, the consumption of chlorine has fallen to 20% from the former level and is presently only 1.5 g m–3. The transparency of river water was in all samples above 30 cm. The background colour of water at site I was due to the presence of organic compounds extremely high, reaching 270°. During our study it was still increasing. The intensive colour of river water is caused by high concentrations of humic substances originating from the drainage basin. Soluble organic substances enter the river water mainly from the marshland and forests at the lower reaches of the river. Most organic substances, formed from remains of plants on the surface of water, are washed into deeper layers of soil or into water during autumn rains. The remainder stays in the frozen soil and is washed into the river during the spring high water after the melting of snow and frozen soil. The pH value of the river water at Paunküla and Vaskjala reservoirs and in the Pirita–Ülemiste Canal

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